PipePy to PipePy

If both operands are PipePy instances, the result is as close as possible to a shell pipe.

from pipepy import git, grep
if git.diff(name_only=True) | grep('readme.txt'):
      print("readme was changed")

If the left operand was previously evaluated, its stdout is passed directly as input to the right operand. Otherwise, both commands execute in parallel and the left's output is streamed into the right.